This invention relates to apparatus and method for monitoring and displaying the tap positions of a load tap changer (LTC).
Load Tap Changers (LTCs) are used in electric power systems to regulate the voltage distributed from substations and along the power lines. An LTC, as used and defined herein and in the appended claims, may be connected in the primary circuit of a power transformer, XFR, as shown in FIG. 1, or in the secondary circuit as shown in FIG. 2. FIG. 1, is a highly simplified version of a prior art system illustrating use of one type of LTC connected in the primary circuit of a power transformer (XFR). In FIG. 1, there is shown the primary (P1) of a power transformer (XFR) to which is coupled the windings 100a and taps 100b of a load tap changer (LTC), 100. Note that in the discussion to follow and in the appended claims, windings 100a, whether connected in the primary or the secondary of the power transformer, may also be referred to as the LTC windings. The LTC may be used to change the effective turns ratio (N1:N2) of the primary and secondary of the power transformer XFR and thereby its output voltage (Vout). The LTC 100 of FIG. 1 is shown to include several taps (T0-TM) which are contacted with a movable contacting element, or contact, C1. The number of taps may vary from a few to many. The movable contact C1 is shown mounted on a tap changer mechanism 105 which is caused to move along the taps T0-TM by a rotatable shaft 103 driven by a motor M1. The shaft 103 can move in a clockwise direction or in a counterclockwise direction and causes contact C1 to advance from tap to tap. For purpose of illustration, in FIG. 1, the contact C1 is shown to be movable in either a down to up direction (from T0 to TM) or in an up to down direction (from TM to T0). In actual systems, the taps may be physically arranged in a circular pattern and the contacting element would then move along a rotary or other suitable path, rather than linearly up and down.
In FIG. 1, the windings 100a, extending between nodes 14 and 16, are connectable in series with the primary windings (P1) of the power transformer XFR. One end 11 of P1 is connected to an input power terminal 17 while the other end 13 of P1 is connected to the top end 14 of the windings 100a. Taps T0 through TM are disposed along the LTC windings, with the lowest tap, To, corresponding to node 16 and the highest tap, TM, corresponding to node 14. For ease of illustration, contact C1, shown mounted on a movable arm depending from mechanism 105, is electrically connected to input power terminal 19 and provides a very low impedance connection between terminal 19 and whichever tap it is contacting. The input power Vin is applied between terminals 17 and 19 and is redistributed via the secondary of the power transformer, XFR, onto output power lines 21, 23. When C1 is connected to tap T0 the primary winding P1 is connected in series with all the windings 100a of the LTC and the effective turns ratio of the primary (e.g., N1) to the secondary (e.g., N2) has been increased. For this condition, the output voltage (Vout) produced at the output of the secondary (SEC1) is decreased. When C1 is connected to tap TM the effective turns ratio of the primary to the secondary is decreased and the output voltage (Vout) produced at the output of the secondary (SEC1) is increased.
In the operation of the system (see FIGS. 1 and 2) the voltage Vout, across the secondary of the power transformer is supplied, via a transformer PT10, to a tap change controller 101 which senses the voltage and produces signals identified as K1 (lower) and K2 (raise). Signals K1 and K2 are applied to the motor M1 and determine whether the motor is driven in a clockwise or counterclockwise direction causing shaft 103 to turn so as to raise or lower tap changer mechanism 105 causing C1 to move along the taps of the LTC windings 100a. If Vout is below some desired level, the controller 101 produces signals (K1, K2) which function to tend to raise Vout to the desired value. Likewise, if Vout is above some desired level, controller 101 produces signals (K1, K2) which function to tend to lower Vout to the desired value.
As noted, motor M1 causes the rotation of drive shaft 103 on which is mounted tap changer mechanism 105 which controls the movement of contacting element C1 along the taps 100b of LTC windings 100a. Mechanism 105 may include gears, cams and switches (not shown) which cause the contact C1 to make contact with the taps in a predetermined sequence.
In the configuration of FIG. 2, windings 100a are connectable in series with the windings of the secondary of the power transformer. As in FIG. 1, which one(s) of the windings 100a get connected in circuit with the secondary windings is a function of which tap is contacted by contact C1. For the condition of contact C1 connected to tap T0, the turns ratio of the primary to secondary is decreased (Vout is increased). For the condition of contact C1 connected to tap TM, the turns ratio of the primary to secondary is increased (Vout is decreased). In FIG. 2, as in FIG. 1, the voltage across the secondary is coupled via a transformer PT10 to a tap change controller 101 which drives a motor M1 which drives a shaft 103a which causes a mechanism 105a to raise or lower the contact C1 to produce a desired Vout. Thus in FIGS. 1 and 2 there is a feedback loop including controller 101 which functions to try to maintain the output voltage at a desired value.
However, a problem exists in that some of the taps may be, or become inoperative. When the system tries to make contact with an inoperative tap, there may be overshoots and/or undershoots, and/or continuous hunting for the desired setting. Known systems do not resolve the problem of identifying “bad” taps and/or any sluggishness and/or delays in the operation and response time of the system including the LTC.
For purpose of ensuring the proper operation of the power distribution system and for maintenance of the power transformer and/or the LTC it is desirable, and/or necessary, to know and display which tap is being contacted, at any time and whether there are problems associated with any of the taps and/or the response time of the system in going from a tap to the next tap (up or down).